Photolithographic processes in the semiconductor industry use raster scanning methods to produce masks. FIG. 1 illustrates a schematic diagram of a known raster-based photolithographic system. One example of a raster-based photolithographic process is an electron beam (e-beam) process. In an e-beam system 102, for example, a reticle 104 is placed on a table 106 which provides a motion to the reticle along a Y axis using a data set 108 and a worktable motion control module 110, and an electronic beam 112 sweeps back and forth along an X axis using the data set 108 and an e-beam control module 114 to provide a raster motion. The system performs raster-based imaging by sweeping the e-beam back and forth along the X axis, turning the e-beam on over designated areas and off until the next designated area, and appropriately stepping the worktable along the Y axis.
Raster-based photolithographic processes are limited to generating only orthogonal line patterns. With respect to an e-beam system, for example, the size of images is limited to integer multiples of the e-beam spot size. The e-beam spot size can be considered to be a pixel of the pattern. A series of stepped images is used to form lines at non-orthogonal angles with respect to a base direction.
FIG. 2 illustrates a stepped angled image formed using the known raster-based photolithographic system of FIG. 1. In this figure, parallel non-orthogonal lines are drawn at an angle of about 45° with respect to the base direction, which functions as a reference. The pattern is built by writing a spot 203 in the X direction, a spot 205 in the Y direction, a spot 207 in the X direction, and so on.
One problem associated with forming non-orthogonal lines using a raster-based photolithographic process is that the non-orthogonal lines require a larger area than the orthogonal lines. Although the minimum horizontal or vertical line width is equal to an e-beam spot size (pixel), the stepped 45° line (a slope of 1:1) requires two pixels 209 and 211, and the space between parallel 45° lines also requires two pixels 213 and 215. In an image containing parallel 30° lines, for example, even more space is required for the lines and the space between the lines.
Another problem associated with forming non-orthogonal lines using a raster-based photolithographic process is that the lines are formed with uneven edges. Although some smoothing of line edges occur during the exposure and development of the mask, the line might not smooth completely depending on the resist sensitivity. The result is an uneven line edge.
Other problems associated with forming non-orthogonal lines using a raster-based photolithographic process involve the use of more metal to form a stepped diagonal line than a minimum width diagonal line. Additionally, writing stepped images which requires a number of e-beam sweeps is less efficient than writing an orthogonal line that requires only one sweep.
Most semiconductor chip layouts are successfully designed using orthogonal lines. When a small number of non-orthogonal lines are required in a layout, they have been formed using stepped images. However, the problems associated with using stepped images to form non-orthogonal lines are exacerbated when a design requires more non-orthogonal lines to be formed in a smaller space.
Therefore, there is a need in the art to provide improved photolithographic techniques to form angled lines.